Oxygen-blown gasification of coal, petroleum coke, residual oil, oil emulsions, tar sands and the like in gasification combined cycle (GCC) power plants generate a large flow of excess nitrogen associated with the production of oxygen feed for the gasification reaction. One difficulty confronting industry is to make the most efficient use of this effectively free excess nitrogen in the GCC facility. Other related difficulties in GCC technology include the continuing drive for more generating capacity at increasingly greater efficiencies and the imposition of severe limits on NO.sub.x emissions to the atmosphere. Finally, as existing GCC facilities' capacity, efficiency, and NO.sub.x emission rates become inadequate to meet increasingly strict future demands, there is a need for inexpensive retrofitting modifications to meet these new, more stringent requirements. The present invention allows the use of the available excess nitrogen in a GCC power plant to increase power generation capacity and improve power generation efficiency with minimum additional capital costs in both new grass roots plants and in re-powering/retrofitting applications.
There are several methods for utilizing nitrogen in a GCC power plant. The simplest method is to vent the excess nitrogen to the atmosphere. The simple venting of the nitrogen is employed when minimum integration or no integration between the air separation unit and the gas turbine is desired.
Another method is described in U.K. Pat. No. 2,067,668. This method consists of returning the excess nitrogen at unspecified temperature to the inlet of the gas turbine air compressor with the sole expressed purpose of reducing NO.sub.x generation. This same method is also disclosed in U.S. Pat. No. 4,297,842.
Still another method is to pass the nitrogen through an auxiliary compressor to inject it into the gas turbine fuel stream which reduces NO.sub.x generation by reducing the flame temperature in the combustor. This practice is disclosed in U.S. Pat. No. 5,081,845, European Pat. No. 0,137,152 and in U.S. Pat. No. 4,224,045. This method requires a costly auxiliary compression system, so, implementation is expensive. Also, since nitrogen injection into the fuel stream typically requires nitrogen pressures of 50-150 psi higher than the 200-250 psia combustion pressure, this practice requires excessive compression with much of the energy wasted as pressure drop to the combustor.
Direct injection of the nitrogen into the combustor of the gas turbine to reduce NO.sub.x is another commonly used method in GCC. U.S. Pat. No. 4,729,217, U.S. Pat. No. 4,707,994, U.S. Pat. No. 4,697,415, U.S. Pat. No. 4,651,519, U.S. Pat. No. 4,631,915 and Canadian Pat. No. 1,238,195 describe the method and several minor variations thereof. This method does reduce NO.sub.x formation, but also requires expensive auxiliary compression to excessive pressures.
Some methods involve heating and compressing the nitrogen before injection directly into the gas turbine expander to generate additional motive flow. U.S. Pat. No. 5,081,845, U.S. Pat. No. 5,076,837, U.S. Pat. No. 4,019,314 and U.S. Pat. No. 3,731,495 by provide a detailed description of these methods. These methods do presumably improve power generating efficiency, but require expensive additional compression equipment and reduce the thermodynamic efficiency of the main expander.
Another scheme for use of the nitrogen stream is described in U.S. Pat. No. 4,697,413. This scheme involves using an auxiliary compressor to feed the nitrogen to the coal gasifier to quench the high temperature reaction.
Other uses for the nitrogen stream involve synthesis of compounds like ammonia that use the nitrogen as a feed stream. These processes all require expensive additional process equipment and add extreme complexity to the overall process so they will not be described here. Some gasification systems employ a small part of the nitrogen to facilitate dry materials transport, but this only uses a small fraction of the available nitrogen, so these systems and their processes will not be described here.
As further background, several processes are known in the art for refrigerating the gas turbine air compressor feed. These processes, which involve expensive, complicated auxiliary equipment, are taught by U.S. Pat. Nos. 3,788,066; 3,796,045; 3,877,218 and 4,424,667.